Nanocrystalline anatase TiO2/reduced graphene oxide composite films as photoanodes for photoelectrochemical water splitting studies: the role of reduced graphene oxide

Abstract

Nanocrystalline TiO₂ and reduced graphene oxide (TiO₂/RGO) composite films were prepared by combining a sol–gel method with hydrothermal treatment, employing titanium isopropoxide (Ti(OⁱPr)₄) and graphene oxide (GO) as starting materials. Although several reports in the literature have explored the benefits of RGO addition in titania films for photocatalysis and water splitting reactions, the role of RGO in the composite is always described as that of a material that is able to act as an electron acceptor and transport electrons more efficiently. However, in most of these reports, no clear evidence for this “role” is presented, and the main focus is deviated to the improved efficiency and not to the reasons for said efficiency. In this study, we employed several techniques to definitively present our understanding of the role of RGO in titania composite films. The TiO₂/RGO composite films were characterized by X ray diffraction, Raman spectroscopy, microscopy and electrochemical techniques. In photoelectrochemical water splitting studies, the TiO₂/RGO_(0.1%) photoelectrodes showed the highest photocurrent density values (0.20 mA cm⁻² at 1.23 V_RHE) compared to other electrodes, with an increase of 78% in relation to pristine TiO₂ film (0.11 mA cm⁻² at 1.23 V_RHE). The transient absorption spectroscopy (TAS) results indicated increases in the lifetime and yield of both the photogenerated holes and electrons. Interestingly, the TiO₂/RGO_(0.1%) film exhibited the best charge generation upon excitation, corroborating the photoelectrochemical data. We proposed that in films with lower concentrations (<0.1 wt%), the RGO sheets are electron acceptors, and a decrease in the charge recombination processes is the immediate consequence. Thus, both holes and electrons live longer and contribute more effectively to the photocurrent density.